The most common type of oven in industrial white bread and bun bakeries is the direct fired oven. The oven can include a traveling tray oven or a tunnel oven. A traveling tray oven has trays loaded with product that are maintained level by three large pitch engineering class roller conveyor chains. The trays travel on the chains toward the back of the oven and go around sprockets to travel down and back toward the front of the oven where the trays are unloaded. The tray oven chains have a bushing every 36″ or 42″ (depending on the size of the oven) to support the center trunnion (axil) of the tray and a third chain with bushings that supports a “Z” bar that stabilizes the tray to keep product from falling off the tray.
A tunnel oven is a long conveyor that has either grids, plates, or mesh attached to two side chains. The side chains travel parallel to each other down the sides of the oven carrying the product. The tunnel oven chains have an attachment at every link to support a grid, plate or wire. The attachment has a keep plate with a keyed hole to fit over a mushroom pin in the attachment. The keeper plate is riveted.
Each type of oven requires engineering class roller conveyor chains. These are heavy duty chains with oversized rollers that support the grids and product and roll along tracks in the ovens. These chains are not the same as roller chain. Engineering class roller conveyor chain is designed for slow moving applications in extreme environments.
Bakeries usually have redundant systems for mixing, make up, packaging, and loading. Most bakeries do not have redundant ovens, mostly due to size and cost. Therefore if the oven does not work, the bakery cannot produce. The most critical component of the oven is the oven chains. If an oven chain fails the oven cannot produce. Replacement oven chains are extremely expensive and usually have very long lead times. Therefore it is essential that the oven chains are well maintained.
Maintenance usually entails regular lubrication and cleaning. The lifespan of an oven chain typically can be from 5 to 10 years depending on the size of the oven, run time, and quality of maintenance. It is possible to have oven chains running for upward of 30 years or more. Conversely, if a chain is poorly maintained the life span can be reduced to 1 to 2 years.
Up until now, oven chains were lubricated based on the bakery production schedule. Chains are usually lubricated once per week on the down day, never knowing if the chain needed to be lubricated every 5 days or every 14 days. Knowing exactly when the chains actually need to be lubricated can significantly extend the life of the chain and reduce the lubrication costs.
When a bakery changes an oven chain, usually the task gets so difficult the project runs the risk of complete failure. Since the replacement costs are so high, $50K to 125K, the plant usually will try to put off replacement for as long as possible.
Typically the bakery has to engage an outside service to conduct the chain replacement. Depending on how bad the chain has worn sometimes the contractor will remove about thirty feet of chain from both sides of the oven and measure their lengths. The measurements are compared to a new length of chain based on the number of links. From that measurement, it is possible to determine the total amount of wear on each chain.
The chain management system disclosed herein will determine and notify the baker when the oven chain needs lubrication. In addition the chain management system disclosed herein will track the length of the chain every time the oven is started and compare it to the chain length when the oven was new. So when the time comes the baker will know how much the chain has worn and plan for the replacement.
There are three key components to the chain management system disclosed herein:    1. Power monitoring to determine when the chain needs to be lubricated and determine the load on the chains.    2. Chain tension monitoring to determine the correct tension and determine when the chain has too much load.    3. Chain length monitoring to determine the amount of wear and if the chain is not running parallel.
Power Monitoring:
Power monitoring is accomplished through the VFD (variable frequency drive) that the oven operators use to control the speed (baking time) of the oven. The operators of the oven will know from experience what the typical loads should be. We also know from experience that once the lubrication begins to degrade the power used to operate the oven starts to increase over time. In the system disclosed herein we can set three set points for the oven power:
Lubrication, when the power reaches a preset wattage an alarm will be set off, indicating that it is time to lubricate.
High Load Warning: When the power reaches a preset wattage an alarm will signal that the drive seeing too much load and something must be wrong.
High Load Alarm: When the power reaches a preset wattage an alarm will signal and stop the oven to prevent a catastrophic chain, grid, tray and pan jam and damage to the oven.
We chose to monitor power instead of amperage for this system because power is linear and has equal sensitivity at both low and high loads, where amperage has no sensitivity for low loads.
Chain Tension Monitoring:
In the past oven chains were kept under tension by the use of large springs. As the springs compress, the amount of tension increases. In addition it is well known that springs fatigue over time and need to be frequently adjusted or replaced. Our chain tension monitor uses hydraulic pressure to maintain a continuous preset amount of pressure to both take up sprockets.
Our engineered class conveyor chain is designed to operate at very high temperatures. The design temperature parameters are 70° F. to 500° F. The linear expansion of this chain is 0.00000633″ (0.000160782 mm) per inch of chain per degree F. Therefore a typical moderate length oven of 152 feet (requires 164 feet of chain per side) will thermally expand 5.370 inches (136.39 mm) from cold to operating temperature. This expansion does not occur all at one time. In the 164 feet of chain expands at 0.01242 inches (0.315 mm) per degree F. At this amount of movement the only way to accurately maintain even continuous tension on the chain is to monitor the hydraulic pressure used to tension the chain.
The chain expands at a minute rate. To maintain the chain tension, we use an air over hydraulic pump. This type of pump operates at 625 psi (43 bar) and can displace 0.83 in3 (13.6 ml) of hydraulic oil per cycle. This minute amount of oil enables the system to maintain precise control over the tension.
Chain Length Monitoring:
A cable actuated position sensor is fastened to the frame of the oven and the sensor wire is attached to the chain take up. The sensor is an incremental encoder with a resolution of 0.025 mm. At this resolution the sensor will signal 12.6 times per F.° of thermal expansion (based on 164′ of chain mentioned above that will thermally expand 0.01242″ (0.315 mm per F°). This is extremely precise. This date goes directly through the PLC to the operator interface to indicate the exact position of the take up, i.e. the length of chain.
Examples of some prior art oven chains are illustrated and described in U.S. Pat. Nos. 4,294,345; 4,657,131; 5,147,033; and 7,086,525.